It is a fair proposition to suggest that the majority of great artists throughout history have been outsiders. Second preposition: as there is such diversity in people, including outsiders, it is unlikely that such people inherently have a special gift. My hypothesis is that outsiders are more likely to be good artists because of their nature as outsiders.
As outsiders have had difficulties in accepting themselves and the world around them, then they have had cause to reflect on this and spend time viewing people and society from the outside. Such thinking allows the outsider to more readily identify specific emotional strands, which they can then express through their art.
This creative process is illustrated by a blog post I read recently. Here the idea is that as people we fill our minds with things, assimilating all this data is confusing and confusion can lead to depression and seeking more data to escape the depression/ find the answer. Expression of thoughts, especially artistically, allows the mind to be emptied or unburdened, to be more like itself and hence more at ease. This idea is counter-intuitive but works: instead of needing more, we need less. Through this emptying process, identification of specific things becomes clear. So, a an artist who is or has become technically competent as an artist is able to express these specific thoughts or emotions through their art.
However, this blog, suggests that through expression of such identifications, the person is released from such burdens, as a path to happiness. Which by the argument above, tends to remove the basis for artisitic endeavour, the outsider is no longer plagued by insecurities about themselves, such emotions are no longer felt, so there is less to express.
However, I would suggest that the artist can do something a realised outsider would perhaps not wish to do. Having discovered how to be happy, why would they wish to be sad again. They can be sad again, I believe it is possible to re-visit any emotional state. Mentally I think it is analogous to places on maps: once you have visited somewhere it is then somewhere known rather than a place on a map, there are memories of the place and how to get there. What is difficult is to revisit somewhere and to feel a similar emotional response on a return visit, i think great artists manage to do this.